Divers must constantly be aware of the physiological changes brought about in their bodies due to increased pressures. Gases are absorbed in body tissues when the body is exposed to increased pressures. Typically, nitrogen is of primary concern to most divers; however, helium or some other inert gas between the alveolar parts and tissues causes the tissues to absorb or desorb gases. The time for body tissues to reach equilibrium with alveolar gas content for a particular gas is the tissue saturation time and is dependent on the tissue half-time. Tissues that have a large blood supply relative to their mass may saturate more rapidly than those with a poor blood supply.
As the tissue inert gas level rises, the partial pressure difference is decreased until equilibrium is reached. During a dive, some of the diver's tissues become saturated and the diver's body will contain more dissolved nitrogen than he normally has when on the surface. During ascent, the dissolved nitrogen must desaturate. The time for desaturation, like saturation, depends on which tissues have been saturated. Therefore, the length of time at a specific depth becomes essential in determining the surfacing rate. A diver who surfaces faster than the body can normally desaturate or eliminate the dissolved gases, will form small bubbles in his tissues. Basically, this is because the circulatory system cannot expel the dissolved gases at the rate at which the external body pressure is decreased. The dissolved gases at this point no longer can stay in solution. The formation of these bubbles is referred to as decompression sickness or "the bends". Decompression sickness can cause permanent injuries or be fatal.
Because of the possibility of "the bends", it is essential that a diver keep an accurate account of his diving status. Presently, most dives are planned prior to entering the water. The Navy Standard Dive Tables are used to calculate the limits for time and depth of dives. Divers can easily plan no-decompression dives or determine necessary decompression stops required to desaturate the body.
There are several apparent disadvantages in using this procedure. Since it would be impractical or impossible for divers to calculate safe limits for dives of varying depths, the diving tables utilize the deepest point of the dive as if the entire time were spent there. As an example, a diver who spends 30 minutes at 90 feet (square dive) must surface or it will be necessary for him to decompress. However, a diver who spends 5 minutes at 90 feet, 10 minutes at 50 feet, 20 minutes at 70 feet, and 5 minutes at 30 feet also must surface in 30 minutes according to the Navy Standard Dive Tables, or it will be necessary for him to decompress. It is obvious that this diver has not reached the same decompression state or dilutant gas tissue partial pressures as the first diver. In this extreme example, the diver would be required to surface sooner than necessary. In most dives, a non-square dive profile is desirable. Therefore, in many instances, divers are required to surface sooner than necessary. The diver-carried decompression computer of this invention continually monitors the diver's decompression status for varying depths.
The decometer would also be valuable to divers who: (1) deviate from the dive plan; (2) operate mixed-gas deep dives; (3) cannot pre-plan due to mission requirements; (4) are on a repetitive dive task; or (5) are working in situations where submerged time is very valuable. The mathematical model followed by the decometer is the same used in calculating the Navy Dive Tables. The Navy's allowable tissue tensions, see Table I, are put in as a look-up table. Each of the current tissue tensions are compared with the table to give the safe ascent output.
TABLE 1 __________________________________________________________________________ MAXIMUM ALLOWABLE TISSUE TENSIONS OF NITROGEN FOR VARIOUS HALF-TIME TISSUES TISSUE HALF TIMES (MINUTES) STORED IN DECOMETER LOOK-UP TABLE DEPTH (FEET) 5 10 20 40 80 120 160 200 240 __________________________________________________________________________ 10 104.280 88.120 71.950 58.400 52.140 50.050 49.790 48.490 46.930 20 126.020 107.360 88.460 72.390 64.910 62.400 62.090 60.510 58.630 30 149.270 127.470 105.340 86.480 77.660 74.710 74.340 72.490 70.260 40 172.660 147.610 122.160 100.450 90.300 86.890 86.470 84.330 81.760 50 195.930 167.620 138.850 114.290 102.800 98.950 98.460 96.050 93.140 60 219.030 187.470 155.390 128.000 115.180 110.880 110.340 107.640 104.390 70 241.960 207.160 171.790 141.580 127.440 122.690 122.100 119.120 115.540 __________________________________________________________________________
Electronically, the instrument senses the pressure of a solid state pressure transducer and inputs this information to a digital micro processor which computes current depth and safe-ascent depth. This information is displayed on a digital readout employing light-emitting diodes. In this fashion, the display indicates even in darkened waters with a minimum chance of misinterpretation. If the mathematical limits of the model which the computer runs are exceeded, the computer is programmed to output a flashing "FU" in place of the safe-ascent depth. Flashing decimal points in all digital positions indicate a low battery condition or safe-ascent depth exceeded.